In the prior art, mica board electric resistance wire heaters (mica board heaters) are well known. U.S. Pat. No. 6,884,974 to Howard et al. is one example of these types of heaters. This patent is incorporated in its entirety by reference. The mica board heater typically has a number of mica boards that are supported by a metal frame. The frame can be square or rectangular in shape and can be made of up elongated frame members that are attached to each other. The mica boards that support the resistance wires extend between two sides of the frame and are generally supported by the frame's elongated members, which are generally in the form of plates.
Heaters in the prior art in one case do not address the problem associated with unacceptable levels of current leakage resulting from moisture adsorption between the layers making up mica support boards. In the second case of the prior art, which addresses the problem of current leakage, the methods taught create a weakened support structure for the heater assembly itself.
The problem is that current leakage occurs between the live metallic heating element wire and ground (or earth) by traveling from this wire along the moist mica to the heater frame and eventually to the metallic frame of the unit using the heater, e.g., an air conditioning unit itself. This phenomenon occurs especially during the cooling season when a unit's cooling A/C coils create moist conditions and the heater is de-energized. Even when switched off, the heater element is electrically alive relative to ground as only one side of the electric circuit is broken to de-energize the element. Leakage current flow may be high enough to create corrosive conditions in the heater wire thus shortening the heater life.
In those prior art heaters that address the problem noted above, each metallic side support member that holds mica pieces in place are riveted to a mica plate that is in turn riveted to the metallic heater assembly main support frame. The resultant effect is that the integrity of the heater depends upon relatively strong metallic members being retained in place by weaker mica material. Consequently, the structural integrity of the heater is compromised.
The type of heater designed to overcome the current leakage problem is shown in FIG. 1, wherein a mica board heater 10 is shown with mica boards 1, resistance wires 3, one frame member 5, and a second frame member 7. The frame members 5 and 7 are separated by a mica board 9, so that the frame 5 is attached, e.g., by riveting, to the mica board 9 and the mica board 9 is attached, e.g., by riveting, to the frame member 7. In this way, the frame member 5 that supports the end of the mica boards carrying the resistance wires is isolated to minimize the problem of current leakage.
The problem with the FIG. 1 assembly is that the mica board 9 becomes a structural part of the frame. Since mica board is inherently weak and clearly weaker than a metal frame, this frame construction can cause problems. That is, if the mica board 9 were to break, the frame attachment between members 5 and 7 would be compromised and the heater could collapse or short out.
Thus, there is a need to improve mica board heaters such that current leakage is minimized but without compromising the frame structural stability.
The present invention solves this need by providing a mica board heater that isolates the mica boards but without compromising the structural soundness of the frame.